(a) Field of the invention
The present invention relates to a field effect transistor, and more particularly it pertains to an improvement in the structure of a field effect transistor adapted for high power and high frequency use and for integrated circuit.
(B) Description of the prior art
Recent developments in field effect transistors have been directed to higher frequency use, higher operation speed, higher output and lower noise. At the present stage of technology, a maximum frequency of several tens giga-Hertz, a highest operation speed of subnano-seconds, a maximum output power of several tens watts in the giga-Hertz band and a minimum noise figure of several dB at several giga-Hertz band are available. All of these characteristics including frequency characteristic, output power, noise figure and so forth are under the influence of the (parasitic) capacitance between the source and the gate C.sub.gs (referred to hereinbelow as source-gate capacitance) and the series resistance r.sub.s, and they will be improved by reducing the series resistance and/or the source-gate capacitance. Here, the series resistance r.sub.s is the resistance from the source to the intrinsic gate (or pinch-off point) in the channel.
Most of the conventional field effect transistors have a long and narrow channel which cannot be pinched off only by the gate bias voltage. In such structure, the series resistance r.sub.s is very large, and hence the drain current-to-voltage characteristic exhibits saturation, with the transconductance g.sub.m being limited by the resiprocal series resistance.
The present invention has proved that such saturating current-to-voltage characteristic is caused by the large series resistance from the source electrode to the intrinsic gate which further increases with the drain current, and has proposed a new type of field effect transistor having a reduced series resistance (of the above definition) and exhibiting unsaturating drain current-to-voltage characteristic (U.S. patent application Ser. No. 817052, IEEE Trans. Electron Devices Vol. ED-22, No. 4, p 185, 1975), which is called the static induction transistor (SIT).
In any type of the field effect transistor, the resistance from the source electrode to the entrance of the intrinsic gate (i.e. the resistance in the portion outside the active current-controlling portion) gives many adverse effects and is preferably reduced as small as possible. Reduction of the source-to-gate distance leads to an increase in the source-gate capacitance. Thus, the reduction of the series resistance r.sub.s and that of the source-gate capacitance are accepted usually as contradicting problems.